1. Field of the Invention
This invention relates to a method for optically recording data in data areas of a disk-like recording medium in which servo signal areas are formed along the track in alternation with the data areas.
2. Description of the Prior Art
Various methods have evolved for recording data on the disk-like recording media using an optical method. Examples of such recording media include those of the so-called read only memory or ROM in which signals are nonrewritably formed by the supplier for supply to users, similarly to digital audio disks such as compact disks (CDs) or video disks, of the so-called programmable ROM or PROM type also known as direct read after write (DRAW) or write-once type in which data can be rewritten only once by the user, and those of the so-called random access memory (RAM) type in which recorded signals can be erased and rewritten, such as opto-magnetic disks. These recording media are characterized by an extremely large recording capacity.
Since these recording media have evolved at different times, signals are recorded on some of these media using distinct signal formats resulting in lack of interchangeability among these different types of the recording media. Since this is troublesome both to the suppliers and the users, the demand for unifying the format has been raised by both the users and the suppliers. As one of the technologies for implementing this unified format, it has been proposed to introduce the concept of the so-called sampling servo according to which servo signals are recorded at a predetermined interval or a predetermined angle on concentric tracks or a spirally extending track on the disk, similarly to the so-called sector servo for the hard disk in the field of the magnetic disk. These discrete servo signals are sampled and held during the rotational driving of the disk to effect a continuous servo control operation. Data signals are sequentially recorded between the adjoining servo signals such that the recording areas for servo signals and those for data signals are alternately provided along the track direction, that is, the scanning direction of the pickup head.
The number of the servo signals formed on the complete perimeter of the disk in the above proposed format is limited by factors such as the rotational speed of the disk or the servo control characteristics. The current practice is to provide one thousand and several hundreds of servo signals, for example, along the complete perimeter of the disk.
It should be noted that the storage capacity of the optical disk is as large as several hundred megabytes, for example, such that it becomes critical to take measures against occurrence of possible errors, such as burst errors. For this reason, the conventional practice is to add error detection or error correction codes to the record data. In copending Japanese Patent Application No. 93892/1986, there has been proposed a data transmission method according to which the data to be transmitted and the additional information affixed to the tail of these data are arrayed two-dimensionally, and first error correction codes are formed for the data of each row, that is, the data extending in one direction, of the two dimensional array, while second error correction codes are formed for the data of each column, that is, the data extending in another direction, for providing a so-called product or matrix code configuration. During recording, data are sequentially read out in the row direction of the product code for sequentially recording the read-out data on the disk.
For purposes of this discussion, it is now assumed that the lengths of the servo signal area and the data area on the disk are equal, respectively, to 2 and 16 bytes in terms of the data numbers, and that the serial length in the row direction of the product code, that is, the length of each row, is equal to, for example, 52 bytes.
In this case, when sequentially recording the data of the product code configuration along the row direction of the respective data areas, the points of data demarcation by the respective servo signal areas are changed from row to row such that there may be caused a situation in which data that may exist across two rows of the product code is recorded in a single data area.
Thus, when a servo signal read error is caused during signal reproduction, even in cases where only one servo signal can not be read correctly so that the data within a single data area has been regarded as the error, the error exists across two rows of the product code, so that there is a lowered error correction ability in the column direction.
It has also been known to use a code system in which each data sector of the disk 1 is arrayed in a two-dimensional matrix of m rows and n columns, the error correction codes are added to each data string in the row direction and the data reading and writing is performed in the column direction. Such a code system is also known as the long distance code or LDC since the length of the data string in the direction of generation of the correction codes, that is the row direction, is rather long, for example, about 100 bytes, the number of interleaves being m which is the number of the columns in the matrix.
When the two-dimensionally arrayed data of the LDC are read out sequentially along the column direction so as to be sequentially recorded in the data areas between the servo signal areas, with the data number in the data area being k, the data sequentially read by groups of m in the column direction are recorded after they are divided at intervals of data lengths k.
In such a case, when the read-out errors of the servo signals are produced during the signal reproduction, and a servo signal cannot be read out correctly so that the data in a particular data area is determined to be in error, the error correcting capability in the row direction of the two-dimensionally arrayed data is lowered.